A sector, called a compass by Galileo, is a hinged calculating instrument with two legs of equal length made of brass, ivory, or wood. On the legs are mathematical scales used in proportion, area problems, volumes, and others. The sector was invented by Galileo Galilei around 1597. The text written on the sector in 1598 by a Thomas Hood from which the tool receives its name was written with no mention of Galileo's instrument and does suggest a parallel discovery in England. The sector is based upon similar triangles. The proportional sides below are a/b = d/c. These distances are applied with a pair of dividers. I have included an online copy of Galileo's book "Operations of the Geometric and Military Compass" to generate interest in this charming tool.
Institute and Museum of the History of Science of Florence, Italy
http://www.imss.fi.it/museo/4/eiv06.html
Edmund Gunter
http://web.mat.bham.ac.uk/C.J.Sangwin/Sliderules/sector.pdf
Excerpt from Heather's Book
http://www.sliderules.clara.net/a-to-z/sector/sector.htm
Ebay! Buy old French and English Design Sectors